Regional scale folding in the Arthur Metamorphic Complex: structural constraints for the Keith River – Lyons River area, NW Tasmania

Cumming, G1, Jackman C.1, Everard, J. L.1 and Gray, D.2

 1 Geological Survey Branch – Mineral Resources Tasmania – Geological Survey Branch, Rosny Park, Australia, 2 Consultant Structural Geologist for Mineral Resources Tasmania – Geological Survey Branch, Rosny Park, Australia

New geological mapping in the Keith River-Lyons River area in NW Tasmania has provided insight into the structural framework of the northern Arthur Metamorphic Complex (AMC) in NW Tasmania. The AMC is flanked to the east by the Oonah Formation and to the west by the Rocky Cape Group. The main lithological units of the AMC share transitional metamorphic, interpreted low angle fault, and both conformable and unconformable contacts. At a regional scale a significant north-plunging synform, or a large north-tilted block, is contained within the high-strain core of the AMC. Five deformation episodes can be observed throughout the area at outcrop-scale. Three early deformational episodes are likely related to the Middle Cambrian Tyennan orogeny, manifested as early, high strain events which caused isoclinal folding and development of schistose axial planar fabric. A rotational shear component, apparent as shear bands, suggests north over south or sinistral transport. Subsequent D3 deformation within the AMC occurred during the later stage of the Tyennan Orogeny. This event folded and tightened the various stacked lithostratigraphic units to form non-cylindrical asymmetric folds. These were later subject to generally northwest-directed, potentially Devonian compression (D4) and tilting. At a regional scale, late-stage north-plunging folds are inferred along the highest strain zone of the northern AMC. This area is a locus for Mesozoic or early Cainozoic faults, and a half-graben also extends along this zone, which is also in the core of the AMC. A late stage (D5) folding event may be partly related to Devonian compression, although the timing and nature of this folding event is largely unclear.


Biography

Grace currently works as a geologist for the Geological Survey Branch at Mineral Resources Tasmania and has spent the last 8 years undertaking mapping work to compile numerous 1:25,000 geological maps of North West Tasmania. 

Late Cambrian – Middle Ordovician extension of the northern Tasmanides: thinned crust that facilitated intense Silurian (Benambran) shortening deformation

Henderson, Robert1, Fergusson, Chris2 and Withnall, Ian3

1Department of Earth and Environmental Sciences, James Cook University, Townsville, Australia, 2School of Earth, Atmospheric and Life Sciences, University of Wollongong, Australia, 3Geological Survey of Queensland, Brisbane, Australia

The Charters Towers and Greenvale Provinces of the Thomson Orogen provide the most extensive exposure of early Paleozoic rocks in the northern Tasmanides. Upper crust represented by the Charters Towers Province developed a thick, Proterozoic passive margin, sedimentary assemblage with a minor contribution from mafic igneous rocks. This assemblage was subsequently overprinted by active margin tectonism commencing in the mid Cambrian (~510 Ma), reflecting a broad scale Tasmanide regime change of that age. The overprinting regime generated thick (>15 km) basinal infill, inclusive of a substantial intermediate to silicic volcanic complement, of the upper Cambrian – Middle Ordovician Seventy Mile Range Group and other upper Cambrian basinal relicts identified for the Charters Towers and Argentine Metamorphics. Magmatic arc plutonism is widely developed as the Ravenswood Batholith and Fat Hen Creek Complex. Regional metamorphism and penetrative fabric development of Early Ordovician age overprinted all of the Proterozoic passive margin assemblage and part of the Cambrian basin fill, with structural analysis favouring its association with extensional strain. Coeval basin formation, plutonism and metamorphism across the province reflects an enduring episode of crustal extension.  

The Greenvale Province consists largely of metavolcanic and metasedimentary tracts with protoliths of Early Ordovician age. Lower Ordovician granites are also represented and in broad aspect, rock units of the province both resemble, and are correlative with, the active margin assemblages of the Charters Towers Province.  Structural analysis shows a deformation history matching that of the Charters Towers Province with the dominant foliation similarly attributed to extensional strain.  Development of the province represents thick basinal infill developed on thinned crust of a continental margin. Contrary to earlier publications, no rocks older than Ordovician are known for the province. Amphibolite and ultramafic units interleaved with metasediments on its eastern (outboard) side suggest it may have developed on oceanic crust.

For both provinces thin crust had developed by the Middle Ordovician, weakening its resistance to subsequent Benambran contraction. Cambro-Ordovician rock assemblages of both provinces were affected by structural/metamorphic overprint with local generation of mylonite in wide shear zones. Contraction is dated as Silurian by Ar–Ar mineral ages, by fabric development in deformed granites of known age and by the age of overlap strata. Tight upright folds and accompanying foliation, generated by shortening, deformed tracts affected by Ordovician extensional strain, steepening previously formed fabrics. Basin fill represented by the Seventy Mile Range Group was inverted. For the Greenvale Province, widespread amphibolite grade metamorphism indicates that much of it has been exhumed from considerable crustal depth (>15km). In contrast exhumation of the Charters Towers Province was heterogeneous, ranging from <5 km (unmetamorphosed Seventy Mile Range Group) to >15 km (amphibolite facies metamorphism and local migmatite). Silurian structural trends, considered by some authors as registering an orocline, are attributed to strain partitioning consequential on oblique convergence.


Biography

Based in Townsville, the presenter has actively pursued research interests in the Tasmanides for over 40 years, with a focus in particular on the Mossman, Thomson and New England Orogens which are extensively developed in Queensland. Mush of this activity has involved collaboration with Chris Fergusson and Ian Withnall.

Early Tasmanides evolution: Passive to convergent margin history in New South Wales, Australia

Greenfield, John1; Gilmore, Phil1; Musgrave, Robert1

1Geological Survey of New South Wales, Department of Regional NSW, Maitland, Australia.

Initial development of the Tasmanides in southeastern Australia involved Early Neoproterozoic rifting/break-up of the Rodinian supercontinent, expressed as tholeiitic dyke swarms and continental rifting in the Adelaide Rift Complex of eastern South Australia. This left highly-extended, transitional crust between the Gawler Craton and Curnamona Province, which became the depocentre for extensive platform carbonate and shallow marine clastic sedimentation. By ~700 Ma, a passive margin developed east of the Curnamona Province which saw the initiation of the palaeo-Pacific Ocean. A final NE–SW phase of rifting in the Late Neoproterozoic was associated with shallow marine platform sedimentation and alkaline magmatism (Mount Arrowsmith Volcanics), further attenuating the eastern Curnamona Province crust and presenting an angular continental salient towards the nascent ocean to the east.

This crustal configuration profoundly influenced the palaeogeography and tectonism that followed during the Delamerian Cycle, as passive margin clastic deposition in the early Cambrian gave way to west-facing subduction in the mid Cambrian. Elements of the resulting Cambrian volcanic arcs (Mount Wright and Loch Lily–Kars) are now immediately adjacent to the Curnamona Province margin. However, regional geological mapping in the Koonenberry Belt has shown that the Mount Wright Arc developed in a rift zone within the Curnamona Province that probably initiated during the last phase of Rodinia break-up in the Late Neoproterozoic. In contrast, the strike-equivalent Loch Lily–Kars Arc was developed in an intra-oceanic setting. Mapping, drillhole and geophysical data shows that this arc segment, along with forearc volcanic rocks of the Ponto Group, were oroclinally folded clockwise almost 90°, and thrust against the southeastern Curnamona Province margin during the Late Cambrian Delamerian Orogeny. If the original arc segments were part of a linear belt, it would have extended southeast from the oroclinal hinge at the Grasmere Knee Zone. Recently acquired and modelled AusLAMP magnetotelluric data show a strong lower crustal conductive anomaly aligned along this trend.

The Delamerian Orogeny caused strong ductile deformation of rocks deposited in the Delamerian Cycle. Areas that were highly attenuated during break-up suffered tight upright folding and oroclinal bending, which may also have been affected by clockwise rotation of the Curnamona Province. Proterozoic Olarian Cycle metamorphic rocks along the margins of the Curnamona Province and the eastern edge of the Gawler Craton also suffered upright open to tight folding during the Delamerian, with σ1 perpendicular to the outboard margin. In the Broken Hill Domain, early southwest-directed fold-thrusting switched to northwest-directed fold-thrust and strike-slip deformation at the end of the Delamerian Orogeny in the Early Ordovician.

Clearly the switch to a convergent setting, with arc accretion and terminal deformation in the Delamerian Orogen, caused extensive shortening of a highly attenuated margin that had been in extension for c. 300 Ma. The Curnamona Province, although acting as a salient in the early Delamerian Orogeny, was itself deformed and could not insulate the distal-foreland Flinders Ranges from deformation late in this event. This had cumulative effects on the ensuing cycles of subduction roll-back, extension and contraction that defined the Tasmanides throughout the Palaeozoic.


Biography

John leads the Geoscience Acquisition & Synthesis Unit in the Geological Survey of NSW, which collects and interprets geoscientific data from geological mapping, geophysical surveys, and specialist studies in mineral deposits, palaeontology, and petrography.

Approaches to structural history in areas of weak to moderate upright folding

Durney, David W1

1Earth & Environmental Sciences, Macquarie University, Australia

A particular style of convergent deformation in upper crustal sedimentary basins in many parts of Australia is gentle to close upright folding on multiple trends, typified by dome-basin interference patterns. This style occurs widely in Siluro-Devonian continental back-arc sedimentary and volcanic strata of the Lachlan Orogen (LO) in New South Wales (NSW). Several hypotheses have been suggested for folding in that region, most notably up to four successive extensional and ‘orogenic’ events.

As conventional methods of structural analysis have limited applicability, the present talk aims to outline the basis of the approach used by Hood et al. at this conference in their analysis of this style in the mildly deformed Quidong Basin in NSW.

The basis is both geological and mechanical. It is geological in the sense that:

  • the area is one of very-low grade metamorphism where the geologically observable solution-transfer/pressure-solution deformation mechanism is known to be prominent in many sedimentary rocks, and
  • extensional deformation is acknowledged in the fromation of the basin.

It is mechanical the sense that:

  • the theoretical rheology of solution-transfer deformation is approximately linear-viscous, meaning slow deformation can occur under very low stress,
  • because of that, ductile rather than brittle deformation is prominent and
  • viscous buckling theory applies to layered strata under horizontal compression.
  • As a separate matter, equilibrium of horizontal forces is recognised as a constraint on likely structural combinations at any given time over broad regions of continental upper crust.

It is suggested that these points also limit the range structural hypotheses that are applicable to such regions.

Briefly, the methods have been selected to emphasise short-period incremental deformations: (1)identification of separate fold directions, (2) application of buckling theory to infer associated shortening directions and their relative timing in upright Ramsay Type 2 or ‘banana’ refolds, (3) correlation of incrementally sensitive mesoscopic spaced cleavages in limestones with the folds, (4) use of outcrop-scale kinematic indicators to establish the 3D character of the deformations, (5) use of buckling theory to explain “fault-related minor folds” and (6) application of force equilibrium to infer regional distribution.


Biography

David Durney began geology at London under John Ramsay (BSc 1965, MSc 1968, PhD 1972), has taught structural geology and mapping (Macquarie University 1973–92, 1993–2005; Barcelona 1992–93; UNSW 2008) and maintains an interest in geology of the Lachlan Orogen.

Structure of the Silurian Quidong Basin: New observations on a microcosm of Eastern Lachlan Orogen tectonic and metallogenic problematica

Hood, David I. A.1, Durney,Dr David W.2, Parkes, Dr Ross A.2

1R&D Dept., Ardex Australia, Seven Hills, Australia, 2Earth & Environmental Sciences, Macquarie University, , Australia

The Quidong Basin is a small (~20 km2) structural basin made up of sediments of Wenlock to Ludlow age within the in-faulted Ordovician–Silurian Tombong Block in the Delegate area of far south-eastern New South Wales. It is a true microcosm of several contested tectonic and metallogenic problems that are common to other parts of the Lachlan Orogen of New South Wales. (1) It displays an angular unconformity with older Silurian sediments attributed to a localised convergent orogeny: the “Quidongan Orogeny”.  (2) There have been competing syngenetic and thrust-related models for the origin of stratbound and faultbound massive sulphide mineralisation in the Basin. (3) It is affected by complex superposed folding and cleavage development whose sequence has not been resolved in the published literature.  (4) Interpreted time relations between folding and prominent faulting in the area have been ambivalent.  (5) The area lacks a direct time constraint on the age of the convergent fold deformation.

A better understanding of the deformational structures and how the unconformity may have formed is also important for palaeogeogrphical reconstruction and stratigraphical correlation of fossil horizons in the faulted and folded sediments. It was for this purpose that DH carried out field observations and analysis of structures at selected sites in the area in support of palaeontological and sedimentalogical work by Ross Parkes for his PhD study at Macquarie University in the early 2000s.

We have since reviewed the structural data of DH to examine their implications for the broader questions listed above. These data have the benefit of being observational and detailed.  The methods of analysis  are well known but not often used in rocks of similar age elsewhere in the Orogen: domain analysis of fold directions and whether a time sequence can be determined in individual superposed folds, and incremental strain-axis analysis of kinematically significant minor fault, vein and stylolite associations.  We also describe a new type of structure—incoherent fault-related folds (DD)—which provides unambiguous criteria to determine the time relation between sub-parallel faults and minor folds.

From these observations and analyses we report (a) the time relation between at least two of the three known fold systems in the area, (b) the same three fold systems above and below the unconformity, which argues against a convergent deformation origin of the unconformity, (c) minor thrust and wrench kinematic data consistent with sinistral wrench reactivation of prominent NNW faults, (d) pre-fold normal movement on the prominent NNW faults, (e) a pre-fold or syngenetic origin of the massive-sulphide mineralisation, (f) no detected map-scale thrusts or inversions and (g) regional correlations which suggest a post-Late Devonian or Kanimblan age of the multiple folding.


Biography

David Hood obtained Hons in Earth Sciences at Macquarie University (1996), specialising in multiple deformations.  Employment includes James Hardie and ARDEX Australia as an Industrial Chemist.

David Durney taught structural geology and geomechanics at Macquarie University to 2006.

Ross Parkes obtained Hons then PhD (2005) in Palaeontology at Macquarie University

Evolution of the Lachlan Orogen in the East Riverina region, NSW: Insights from 100 new SHRIMP dates

Bodorkos S1, Gilmore PJ2, Eastlake MA2, Bull KF2, Blevin PL2, Trigg SJ2, Campbell LM2 and Waltenberg K1

1Minerals, Energy and Groundwater Division, Geoscience Australia (GA), 2Geological Survey of New South Wales (GSNSW), Department of Regional NSW

The southern part of the central Lachlan Orogen in NSW is prospective for intrusion-related tin-tungsten (e.g. Ardlethan), porphyry-style copper-gold, and orogenic gold mineralisation; however, the regional geological framework has long been poorly understood. GSNSW’s 2014–2018 East Riverina mapping project (spanning the area between West Wyalong and the Murray River, east to Cootamundra and Adelong, and west to Narrandera and the Murray Basin) updated large areas of 1960s–1990s geological mapping, to support mineral prospectivity studies. The project included a collaborative GA–GSNSW geochronology program that generated nearly 100 new U–Pb SHRIMP zircon dates, to establish a regional chronology of felsic and intermediate magmatism, and to understand the depositional history of the intervening basin successions. Some highlights include:

  1. Sheared 493 Ma and 489 Ma granites of the Belimebung Igneous Complex confirm previous LA-ICPMS dating of the first Cambrian igneous rocks identified within the Gilmore Fault Zone. They were intersected in drillcore beneath the early Silurian Gidginbung Volcanics north of Barmedman, and indicate a possible latest Cambrian age for NW-trending magnetic lineaments near the Gilmore Fault.
  2. The propylitically-altered 439 Ma Cooba Monzonite southeast of Junee extends the known distribution of ‘Phase 4’ Macquarie Volcanic Belt rocks (prospective for porphyry copper mineralisation) to the south.
  3. Tholeiitic andesites of the 432 Ma Junawarra Volcanics and related rocks host gold mineralisation at Dobroyde, northeast of Junee. These are geochemically distinct from, and significantly younger than, the shoshonitic 439–436 Ma Gidginbung Volcanics to the northwest, and establish two separate associations of Silurian gold-bearing volcanic rocks in the region.
  4. More than 20 S-type plutons of the 432–427 Ma Koetong Supersuite have been dated, as far north as Nymagee, east to Young and Tumut, and west under Murray Basin cover to Barellan and Howlong.
  5. The predominantly mid-Silurian ‘Wagga Batholith’ is transected and flanked by NW-trending belts of Early Devonian (420–412 Ma) plutonic rocks linked to intrusion-related mineralisation. S-type granites associated with tin-tungsten mineralisation include the 418 Ma Burrandana Granite and the de-silicated 417 Ma Ryan Granite in the south, and fractionated 415–413 Ma granites near Ardlethan in the north. Contemporaneous I-type rocks include the cassiterite-bearing Yithan Rhyolite at Ardlethan, diorite associated with gold mineralisation at Mount Adrah, and intrusions east and south of Tarcutta and Holbrook.
  6. Regional Early Devonian volcanism encompasses the expanded 419–416 Ma Gurragong Group in the Cargelligo-Ardlethan area, and the newly-defined Culcairn Group (comprising the 415–413 Ma S-type Budginigi Ignimbrite and the 413–411 Ma Wallandoon Ignimbrite and Hickory Hill Diorite) in the Culcairn-Henty-Walbundrie area. These volcanic units constrain the timing of pre-eruptive siliciclastic sedimentation and influence the detrital signatures of post-eruptive sediments.
  7. I-type plutonism is youngest in the southwestern East Riverina. Early Devonian (414 Ma and 411 Ma) Leeton Igneous Complex granites stitch the unexposed northwestern extension of the Kancoona Fault. Northeast and north of Albury, the 407–402 Ma Mullengandra Monzodiorite and Jindera Granite appear to be associated with Central Victorian granites of similar age.

Biography

Simon Bodorkos has worked in Geochronology at Geoscience Australia since 2007, and has conducted the U-Pb SHRIMP dating program supporting the Geological Survey of New South Wales’ East Riverina project since 2013. His co-authors include GSNSW’s East Riverina mapping team and mineral systems specialists.

Apatite as as indicator of porphyry fertility in the Northparkes district

Wells, Tristan1, Meffre, Sebastien1, Cooke, David R,1,2, Steadman, Jeffrey A.1, Goemann, Karsten

1CODES. Centre for Ore Deposits and Earth Sciences, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia. 2ARC Industrial Transformation Research Hub for Transforming the Mining Value Chain – TMVC, Private Bag 79, University of Tasmania – Hobart, Tasmania, 7001. 3Central Science Laboratory, University of Tasmania, Hobart Tasmania, 7001

The resistate nature of apatite in the weathering profile, combined with its potential to record physical and chemical information about magmatic and hydrothermal systems, makes it a useful mineral for assessing magmatic fertility. Magmatic apatite trace element compositions can reflect the degree of fractionation, sulphur content and oxidation of the host rock, whereas hydrothermal or recrystalised apatite also has the potential to record hydrothermal fluid evolution.

Apatite from the Northparkes district, NSW were analysed by colour cathodoluminescence, scanning electron microscopy, electron micro-probe and laser ablation inductively coupled mass spectrometry. Colour cathodoluminescent imaging of apatite from Northparkes highlights complex zonation and differing luminescent colours that are linked to variations in mineral trace element abundance and crystal origins. At Northparkes, magmatic apatites have a lavender or blue luminescent colour, whereas apatites that have complex internal geometries and a yellow-green to brownish luminescent colour are associated with hydrothermal alteration and proximity to mineralised centres. Magmatic and hydrothermal apatite have similar crystal forms, making them virtually indistinguishable from each other without the aid of colour cathodoluminescence. Depletion of light rare earth elements in apatite is associated with hydrothermal alteration across the Northparkes district. Hydrothermally altered apatite from the mineralising intrusions at the Endeavour 26 deposit have pronounced LREE depletion and MREE enrichment, concurrent with a strong positive Eu anomaly. The detection of apatite with these characteristics can be used to infer the proximity to mineralization in porphyry systems in the Northparkes district.


Biography

Tristan is a part-time PhD student researching the fingerprint of magmatic fertility in the Northparkes district and broader Macquarie Arc. His research uses a combination of whole rock and mineral chemistry to define the signature of fertile porphyry intrusions and vectors towards them.

Orogenesis terminated by mafic underplate delamination at prior passive rift margins: The Delamerian-Ross example

Foden, John1, Tappert, Ralph1, Todd, Angas1, Segui, David1

1Department of Earth Sciences, University of Adelaide, Australia

Stretching from southern Africa to north east Australia the Late Neoproterozoic to Late Cambrian aged Delamerian – Ross Orogen formed at the rifted Rodinia break-up margin, facing the newly opened Pacific. The orogenic history of this margin reflected initiation of subduction of the Pacific plate. At the end of the Cambrian, along the entire belt, active convergent orogenesis was terminated abruptly by rapid exhumation, uplift and cooling. This event is recorded as a widespread regional Upper Cambrian unconformity from southern Africa across Antarctica and into Tasmania. Rapid erosion that resulted from this event produced latest Cambrian to Early Ordovician proximal and distal siliciclastic sediment deposits including conglomerates and fluvial sandstones as well as marine turbidites. These deposits include the South African Cape Supergroup, the Ross Orogen Carryer and Douglas Conglomerates, the Tasmanian Jukes and Owen Conglomerate and the western Victorian turbidites.

In South Australia Jurassic aged kimberlite intruded the Delamerian Orogen and transported an abundant population of mafic xenoliths ranging from garnet-pyroxenite (‘eclogite’) to pyroxenite and mafic granulite. Mineral assemblages include; Cpx-Gt-Rt, Cpx-Gt-Amp-Rt, CPX-Gt-Amp-Ky, Plag-Cpx-Gt± Amp, Ky, Il. These were sampled from lithospheric mantle recording pressures in the range 8 to 25 kbar. Exsolution of garnet and kyanite from Cpx provides evidence for cooling at constant or increasing pressure. Whole rock Nd isotopes yield an imprecise Late Neoproterozoic external isochron and their geochemical composition indicate that parental mafic magma was anorogenic rift-related tholeiite. Importantly the suite of samples forms clear compositional trends that show igneous ‘gabbroic’ pyroxene + plagioclase fractionation control even though many samples are now plagioclase-free.

The conclusion is that these were magmas produced during Rodinian rifting and breakup and formed underplated gabbros at Moho depth. Subsequent cooling to produce plagioclase-free, garnet and pargasite -bearing assemblages lead to increasing density and subsequent delamination resulting in buoyant crustal uplift and probably coupled with slab roll back led to orogenic termination. The common occurrence of high pressure pargasitic amphibole may implicate the role of hydrous flux from the subducting Pacific plate in catalysing high pressure cooling reactions in the mafic underplate. Critical to their density increase, P-T modelling of the pyroxenite bulk compositions indicates that at Moho depths (9kbar) cooling of the Neoproterozoic magmatic underplate would cross the garnet and pargasite-in reactions at 1000oC and the plagioclase-out reaction at 750oC. The time taken for initial mafic magmatic intrusions at 1350oC to cool to cross these important density increasing reactions at a Moho T of ~600oC is of the order of 5-15 m.y. Delamination may also promote local thermal convection leading to anomalous asthenospheric ascent and the production of post-tectonic magmas.

The style of orogenic termination described here forms a distinctive class and reflects subduction at the rift magma-rich margins of continental fragments formed during break-up of earlier continents. This orogenic style seems common to many belts formed during Gondwanan accretion.


Biography

Emeritus Professor John Foden is an igneous petrologist and geochemist with a lengthy history of teaching, research  and post-graduate supervision at the University of Adelaide . His specialist interests include; magma generation and differentiation, modern subduction magmatism in the Indonesian Sunda Arc, the orogenic history of the early Palaeozoic margin of Australia, the use of Fe-isotopes in the interpretation of high temperature processes and kimberlite and diamonds.

Unravelling the Tumut Trough: A Middle Ordovician age for the Brungle Creek Metabasalt, eastern Lachlan Orogen

Bruce, Michael,1 Percival, Ian1, Zhen, Yong Yi1

1Geological Survey of New South Wales, W.B. Clarke Geoscience Centre, Londonderry, Australia

Alpine-type ultramafic bodies are exposed in numerous localities throughout the Lachlan Orogen of New South Wales. Despite the tectonic significance of such oceanic lithosphere to the development of the orogen, few studies on the genesis of these bodies have been documented.

The Coolac Serpentinite is an Alpine-type ultramafic intrusion that marks the eastern edge of the Tumut Trough in the eastern Lachlan Orogen. Recent petrological, geochemical and geochronological studies into the massive harzburgite (Bruce 2018) that makes up most of this body reject any ophiolitic association with rocks of the North Mooney Complex. These rocks are traditionally ‘lumped in’ as part of the proposed Coolac Ophiolite Suite, largely because of their physical location and resemblance to a layered, crustal ophiolitic sequence. Instead, a 2-stage melting model is proposed for the origin of the Coolac harzburgites with a late Cambrian latest melting event inferred from an allochthonous block (501 ± 2.6 Ma; U/Pb zircon) with petrological links to the harzburgites.

The ‘block’ has been incorporated into the Silurian Jackalass Slate within the trough, which was previously thought to be simple sedimentary trough fill, but is now partly interpreted as a sedimentary-matrix melange incorporating much older blocks. This interpretation is supported by blocks of chert found within the Jackalass Slate that contain conodont elements (Periodon aculeatus) of late Darriwilian to early Gisbornian age. Slightly older conodont assemblages with diagnostic elements of Periodon hankensis, indicating a late Dapingian to early Darriwilian age, are also found within chert lenses of the structurally underlying Brungle Creek Metabasalt.

Chert blocks within the Jackass slate and chert lenses within the Brungle Creek Metabasalt show near-identical, REE chondrite normalised abundances and patterns as well as evidence of significant terrigenous mafic volcanic and hydrothermal input. This implies that the Brungle Creek Metabasalt is coeval with chert deposition and is thus early Middle Ordovician in age (465−468 Ma). The presence of Cambrian basement, widespread cataclastite in and around the Brungle Creek Metabasalt and structurally underlying Bullawyarra Schist and identical chert units in the Brungle Creek Metabasalt and younger Jackalass Slate, as well as lenses of chert−volcanic clast conglomerate within the Brungle Creek Metabasalt, all support the structural interpretation of Stuart-Smith (1990), who suggested uplift, collapse and extension along a low-angle detachment fault. In addition, it is suggested that older, collapsed blocks have later been re-sedimented into younger Silurian basin strata.

Bruce M.C. 2018. Petrology, geochemistry and a probable Cambrian age for harzburgites of the Coolac Serpentinite, New South Wales, Australia. Australian Journal of Earth Sciences 65, 335−355.

Stuart-Smith P.G. 1990. Evidence for extensional tectonics in the Tumut Trough, Lachlan Fold Belt, NSW, Australia. Australian Journal of Earth Sciences 37, 147−167.


Biography

Michael Bruce currently works for the Geological Survey of NSW, Department of Regional NSW. Michael joined this organisation in 2005, having completed a PhD at the University of Queensland. His interest includes petrology and geochemistry of igneous rocks but is particularly focused on the genesis of mafic and ultramafic rocks.

Life Cycle of the Ordovician Macquarie Arc, Lachlan Orogen, Eastern Australia

Qing Zhang1, 2, Allen Nutman1, Solomon Buckman1

1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China

The Ordovician intra-oceanic Macquarie Arc is preserved in a tectonostratigraphic terrane, faulted to the west and east against coeval, quartz-rich turbidites of the Adaminaby Group, within the Lachlan Orogen of eastern Australia. Debates exist concerning the allochthoneity of the Macquarie Arc, the polarity of its related subduction and the nature and exact timing of collision with Gondwana. These key problems are addressed by the integrated application of field observations, petrography, zircon U-Pb-Hf isotopes and whole rock geochemistry to key units within the Macquarie Arc stratigraphy. By these approaches, it has been possible to answer (i) the timing and juvenility of the arc initiation, (ii) the timing of arc-continent collision, and (iii) the allochthoneity and emplacement mechanism of the Macquarie Arc onto the eastern edge of Gondwana. The new results confirm that the Macquarie Arc was initiated far from the continent with no continental contamination, most likely via outboard (eastward) subduction at high dip angle. The arc started colliding with the eastern Gondwana during the Late Ordovician (~456 Ma), indicated by the trench-fill sedimentary protoliths of the Triangle Formation. Preservation of juvenile island arc on continental margins is aided by outboard subduction that results in emplacement of the arc complex as a klippe in an upper plate position on top of the passive margin sequence, instead of an autochthon extending deep to the mantle, amalgamated with the continent through a back-arc closure.

These results enriched the knowledge of continental growth of eastern Gondwana that it involved the episodic addition of juvenile oceanic terranes via east-dipping subduction, and emphasized that the detrital zircon ages could record the process. By establishing the arc chronology via these zircons is a major contribution to understanding the geodynamic setting of this Paleozoic arc-related copper mineralization along the Pacific margin.


Biography

Qing Zhang is a postdoc at the Institute of Geology and Geophysics, Chinese Academy of Sciences (2020.7-), working with the SIMS research group. She is interested in understanding the evolution of orogens, with particular interests in using zircon U-Pb ages and Hf-O isotopes methods.

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